Numerical simulation of heat and mass transfer in vacuum far-infrared drying of rhubarb slices enhanced by multi-frequency ultrasound.

Ultrasound-enhanced drying technology is an emerging green and energy-saving technique that can effectively improve drying quality while enhancing drying efficiency. However, the enhancement mechanism of multi-frequency ultrasound in vacuum far-infrared (V-FIR) drying remains unclear. This study develops a coupled heat and mass transfer model for the multi-frequency ultrasound-enhanced vacuum far-infrared (UEV-FIR) drying of rhubarb slices to analyze the heat and mass transfer mechanisms during the drying process. Through simulation, the variations in temperature, moisture content, and other parameters of rhubarb slices under single-frequency ultrasound (28 kHz) and multi-frequency ultrasound (25 kHz + 28 kHz + 40 kHz) were investigated. The three ultrasound frequencies were applied in a concurrent excitation mode. Furthermore, the changes in material temperature, moisture concentration, and acoustic pressure during vacuum far-infrared drying and UEV-FIR drying were compared. Finally, the simulation results were validated against experimental data. The results indicate that the proposed model adequately describes the drying process of rhubarb slices under multi-frequency UEV-FIR drying. Within the tested 25-40 kHz range, sound pressure amplitude increased with ultrasonic frequency under single- and multi-frequency coupled excitation. Furthermore, a multi-frequency ultrasonic field exhibits a higher maximum sound pressure and an extended spatial influence compared to a single-frequency counterpart. Multi-frequency ultrasound has a significant enhancing effect on heat transfer and moisture migration in rhubarb slices. This study lays a solid foundation for the optimization of processing techniques and the development of equipment for other traditional Chinese medicinal materials, and also provides a theoretical basis for the application of UEV-FIR technology.